The present invention relates to dispersions of conductive nano-fillers (particularly carbon-based conductive nano-fillers) in polymeric matrices (particularly epoxy resin systems) and composites produced therefrom, and methods for their production. The present invention further relates to an improved method for dispersing such nano-fillers in polymeric matrices and in composite structures by the use of a soluble thermoplastic polymer to deliver the nano-fillers into the polymeric matrix. The present invention also relates to composite materials and structures having improved electrical and electromagnetic performance.
Composite materials comprising polymer resins and fillers are well-known as structural materials for their capability to absorb loads and stresses, and offer advantages over metals and ceramics in that the polymer composites are lightweight, easier to manufacture and readily tailored for specific applications, and exhibit high specific stiffness and strength and a low coefficient of thermal expansion. However, the application of these materials to modern aircraft primary and secondary structures presents special challenges due to the dielectric nature of the resin matrix. Composite materials and/or structural modifications are usually required to fulfil the stringent functional and certification requirements of such components in terms of electric and electromagnetic (EM) properties.
The primary structures for modern aircraft should provide capability for lightning strike protection, potential discharge, electrostatic dissipation (ESD), electromagnetic interference (EMI), electrical grounding and electromagnetic shielding. In order for a carbon fibre-reinforced polymer material to achieve such characteristics, it is necessary to achieve a homogeneously conductive material able to improve charge dissipation. Moreover, the most recent use of “third generation materials” in such structures means that improvements are required in the z-direction conductivity of the composite to avoid potential catastrophic accidents caused by the ignition of fuel vapours in the aircraft wing tanks as a result of a lightning strike event. As used herein, the “z-direction” refers to the direction orthogonal to the planes on which the reinforcing fibres are arranged in the composite structure or the axis through the thickness of the composite structure.
Thus, composite structures with tailored electromagnetic properties are required in several applications where it is necessary to control the EM radiation propagation, reflection, absorption and transparency. Aircraft composite structures require high electromagnetic shielding effectiveness to reduce the disturbances caused by external sources such as lightning strike, high-intensity radiated fields (HIRF) and electromagnetic pulses (EMPs) on the on-board systems. The primary shielding mechanism in aircraft structures is reflection and the efficiency is normally a function of the electrical conductivity. Composite structures with EM radiation absorption properties are also required for low observable structures and components used in aircrafts, warships, submarines, missiles and wing turbine blades. State of the art radar absorbing materials (RAMs) are generally based on polymeric compounds containing high loadings of relatively heavy metals or alloys.
An alternative approach can be based on single or multi-layered radar absorbing structures (RAS) designed to reduce the EM radiation reflection through destructive interference mechanisms. Typical examples of such class of structures are Dallenbach layers, Salisbury Screens and Jaumann layers.
Reflectivity can be reduced by designing structures with specific thicknesses using materials with tailored complex permittivity (c) and permeability (μ) properties.
The ability of a material to absorb energy is a function of the ratio between the imaginary and the real parts, or equivalently of the loss tangents, in accordance with the following expression:tan δε=ε″/ε′ tan δμ=μ″/μ′
For dielectric material modified by conductive fillers, the absorbing capability can be enhanced through its complex permittivity modification, which at microwave frequencies is closely related to the electrical conductivity.
Carbon nanotubes (CNTs) are a class of conductive fillers which can be used to modify electric and electromagnetic characteristics due to their low density, excellent electrical conductivity, as well as excellent mechanical properties, high thermal conductivity and high thermal stability. However, the use of CNTs in composite materials has been limited due to problems in dispersing them effectively in the composite matrix, which is critical to the improvement of the conductivity of the matrix, and to do so without jeopardizing thermo-mechanical performance. The onset of electrical conductivity in a CNT composite occurs when a critical filler content (referred to as the “percolation threshold”) is reached and the conductive fillers form a conductive pathway. As used herein, the percolation threshold is the filler content to achieve conductivity values in direct current (DC) conditions equal to or greater than 10−6 S/m.
Various attempts have been made to compatibilize and disperse CNTs in polymeric matrices. US-2004/0186220-A discloses a method for compatibilizing and dispersing high contents of single-walled carbon nanotubes (SWCNTs) in electrically insulating matrixes using a solvent-assisted procedure, and manufacturing composite fibres, films and solids through a polymer coating/wrapping process. US-2010/0009165-A, US-2008/0194737-A, US-2008/0187482-A and US-2006/0054866-A describe the functionalization of CNTs in a “non-wrapping” fashion using functionalized conjugated polymers such as polyarylene ethynylenes, which are then dispersed in thermoplastics and thermosets. WO-2010/007163-A reports the use of a CNT-modified binder to coat structural fibres in composite structures, which purports to solve manufacturing issues connected to the viscosity of highly CNT-loaded systems, especially for infusion applications. US-2009/0298994-A discloses a CNT dispersion process in a polyolefinic matrix comprising the in-situ polymerization of a polyolefin on the nanofiller surface and the subsequent dispersion of the obtained material in the matrix. US-2010/0189946-A reports the use of a combination of linear and functionalized/grafted fluorinated polymers to compatibilize CNTs in polymer matrices. US-2009/0176924-A discloses a method to produce highly concentrated CNT pulverulent master-batches in various polymers. WO-2009/147415-A reports a material comprising at least one thermosetting resin, carbon conductive additive materials and at least one thermoplastic polymer resin that dissolves in the resin and phase separates upon cure. In addition, there are various commercially available, but relatively expensive, CNT products in which the CNT is pre-dispersed in a resin precursor system, for instance CNT/epoxy masterbatches, which are reported as achieving optimum dispersion levels in an epoxy or epoxy blend resin system.